With (un)set_user_preference, you can either pass a $user object, or a $userid, to say which user's preference to set. (If not passed, it defaults to $USER).

Once the preference is set (or unset), it updates the cache in $user->preference cache.

The case that is missed is that if you pass $USER->id to the function as an integer, then it dose not updated the cache in $USER. I think it should, and it took me a while to debug. Therefore, I am going to make a patch.